Conventional microlithographic exposure systems utilizing a charged particle beam are classified into the following three types:
(1) spot-beam exposure systems PA1 (2) variable-shaped-beam exposure systems PA1 (3) block exposure systems
Compared with single-shot projection-exposure methods using conventional light, the charged-particle-beam (CPB) exposure methods listed above offer tantalizing prospects of vastly improved resolution but tend to exhibit disappointingly very low throughput. In particular, exposure methods (1) and (2) involve tracing the pattern feature-by-feature using an electron beam having an extremely small spot diameter or square-shaped transverse profile; consequently, throughput is extremely low with these methods.
Block exposure systems were originally developed with the object of improving throughput over what was achievable with the spot-beam and variable-shaped beam systems. With block exposure systems, throughput is improved by forming a predetermined pattern on the reticle and batch-exposing the pattern. However, the number of patterns formed on the reticle is limited to a small number. Also, a complete die pattern usually cannot be transferred using a block exposure system, and another system (typically a variable-shaped-beam exposure system) must be employed to complete transfer of the entire die pattern. Consequently, the block exposure system has exhibited disappointingly low throughput.
In order to further improve throughput, much effort is currently being expended to develop CPB exposure apparatus based on the so-called "divided" projection-transfer system, which projects and exposes discrete portions of the reticle pattern in a batch manner onto the substrate surface.
Reference is now made to FIGS. 8(A)-8(C) and 9 that depict a conventional CPB microlithograpic exposure apparatus based on the divided projection-transfer system. FIG. 8(A) is a top view of a conventional semiconductor wafer W showing multiple dies (a representative die is denoted by the hatched lines). FIG. 8(B) depicts one die, which is divided into four "stripes" S.sub.1 -S.sub.4 (a representative stripe S.sub.1 is denoted by the hatched lines) that are individually and sequentially projected in a scanning manner. The stripes are separated from each other by boundary lines BL extending in the x-direction. (The boundary lines BL are shown for convenience; the boundary lines BL are not actually present on the wafer.) FIG. 8(C) depicts a typical division of a stripe S into multiple "subfields" SF (a representative subfield is denoted by the hatched lines) that are individually and sequentially projected typically in a raster manner. The subfields SF are separated from each other by boundary lines BL' extending in the y-direction and boundary lines BL' extending in the x-direction.
For exposure, the wafer W is normally mounted on a wafer stage (not shown in FIGS. 8(A)-8(C)). During exposure of the reticle pattern, the wafer stage is step-shifted in the y-direction and scan-shifted in the x-direction. Meanwhile, the charged particle beam is deflected in both the x-direction and the y-direction as required to expose each subfield.
After all the subfields in a first stripe S.sub.1 are projected, the wafer stage is step-shifted in a y-direction to position the wafer W for exposure of the next stripe S.sub.2. During exposure of each stripe, the wafer stage is scan-shifted in an x-direction at a constant velocity as noted in FIG. 8(B). As shown in FIG. 8(C), during exposure of a stripe S, the charged particle beam is deflected in the negative y-direction to batch-expose each of the subfields in the first column C.sub.1. The charged particle beam is then deflected in the positive y-direction to batch-expose each of the subfields in the second column C.sub.2. The charged particle beam is then deflected in the negative y-direction to batch-expose each of the subfields in the third column C.sub.3. Next, the charged particle beam is deflected in the positive y-direction to batch-expose each of the subfields in the fourth column C.sub.4. This order of exposure is continued for the remaining columns of subfields. That is, the charged particle beam is deflected in the negative y-direction to expose the subfields in each odd-numbered column, and deflected in the positive y-direction to expose the subfields in each even-numbered column.
For example (FIG. 8(C)), as the charged particle beam is exposing the subfields in the first column C.sub.1 of the stripe S, the wafer stage is scan-shifted at a constant velocity in the positive x-direction. Thus, the deflection direction of the charged particle beam is (-y, +x) in the figure. In moving from the first column C.sub.1 to the second column C.sub.2, the charged particle beam is shifted in the -x direction. The charged particle beam is then deflected in the (+y, +x) direction to expose the subfields in the second column. In moving from the second column C.sub.2 to the third column C.sub.3, the charged particle beam is shifted in the -x direction. These motions are repeated as exposure progresses along the stripe S to expose all the subfields SF in the stripe.
After the first stripe S.sub.1 is exposed, the wafer stage is shifted by one step (approximately equal to the width of one stripe) in the -y direction to position the wafer for exposure of the second stripe S.sub.2 as shown in FIG. 8(B). The wafer stage is then scan-shifted in the -x-direction as each subfield in the second stripe S.sub.2 is exposed.
It is noted that each die can have one or multiple stripes, and each stripe typically includes multiple subfields.
FIG. 9 shows the relationship between the reticle M and the sensitive substrate ("wafer") W. More specifically, FIG. 9 depicts a portion of a stripe S on the reticle M and a corresponding region on the wafer W. The depicted region on the wafer W comprises multiple subfields SF each having a corresponding subfield RSF on the reticle M. The reticle subfields RSF are separated from one another in both the x-direction and the y-direction by respective light-shielding boundary zones BZ. Each boundary zone BZ includes a strut (not shown, but understood to extend along the respective boundary zone in the respective x- or y-direction). The struts collectively provide physical support for the reticle subfields RSF. The boundary zones BZ surrounding each reticle subfield RSF also function as a respective field stop.
Continuing further with FIG. 9, a charged particle beam B illuminates each of the reticle subfields RSF in a sequential manner. A portion of the charged particle beam B passes through the illuminated reticle subfield RSF. The portion passing through the illuminated reticle subfield has an ability to form an image of the illuminated reticle subfield on the wafer surface. A projection system comprising lenses PL1 and PL2 forms the image of the illuminated reticle subfield RSF on a corresponding subfield SF on the surface of the wafer W. The magnification imposed on the image by the projection system is usually negative, for example, -1/4. Therefore, the direction of motion of the reticle stage (not shown but understood to hold the reticle M) is opposite to the direction of motion of the wafer stage (not shown but understood to hold the wafer W) in the figure. For example, the wafer W is moved in the positive x-direction whenever the reticle M is moved in the negative x-direction, and the wafer W is moved stepwise in the positive y-direction whenever the reticle M is moved stepwise in the negative y-direction.
As discussed above, the reticle subfields RSF are separated from each other by respective boundary zones BZ. However, the corresponding subfields SF on the wafer W are contiguous. Therefore, if the magnification power of the projection system is -1/4, then the scanning velocity of the reticle stage is more than four times the scanning velocity of the wafer stage. For the same reason, the distance over which the reticle stage moves stepwise is more than four times the distance over which the wafer stage moves stepwise.
Thus, with conventional divided projection-exposure apparatus, each of the reticle subfields RSF is exposed as a single shot and all the reticle subfields RSF to be exposed are defined on the reticle. As a result, conventional divided projection-exposure apparatus achieve a significantly improved throughput compared to the conventional spot-beam exposure, variable-shaped-beam exposure, and block exposure systems. However, conventional divided projection-exposure apparatus require that the reticle pattern be divided into multiple subregions (e.g., subfields) and that transfer of the reticle pattern be performed subregion by subregion. Thus, the accuracy with which the projected subregions must be "stitched" together contiguously on the wafer surface is very high. Actual results achieved using conventional divided projection-exposure apparatus indicate that the required stitching accuracy is frequently not achieved, resulting in unwanted displacements of the projected subregions relative to each other.
Moreover, conventional divided projection-exposure apparatus generally exhibit a phenomenon called the "Coulomb effect". One type of Coulomb effect arises because the beam current passing through a reticle subfield RSF typically varies from subfield to subfield depending upon the feature density of each of the illuminated reticle subfields. Thus, the magnitude of such a Coulomb effect is typically different from one subfield to the next, resulting in a corresponding variation (from one subfield to the next) in the position along the z-axis of the focal point of the subfield images as projected onto the wafer W, and consequent blurring of the transferred images. To minimize this phenomenon, the projection system must be carefully controlled to ensure a constant focal length from one reticle subfield RSF to the next.